Conventionally the nose cone is attached to the fan hub by a number of circumferentially spaced, axially extending bolts disposed around the base portion of the cone and engaging the fan hub assembly. To provide access to these bolts, for fitting or removing the nose cone, elliptical holes are provided in the nose cone. Alternatively pockets or indentations below the nominal surface of the cone are formed in the nose cone base region. The bolts are fitted within these pockets which are large enough to accommodate the heads of the bolts.
A problem with these arrangements is that the airflow into the fan, over the nose cone, is disturbed, in particular in the region surrounding the fan hub at the root portion of the fan blades. In the first arrangement some air flows into the elliptical holes and also the sharp edges of the holes affect and disturb the remainder of the flow of air passing over them. In the second arrangement there are more rounded edges to the pockets than are found with the holes reducing the effect they have on the airflow passing over them. However, the pockets are larger than the holes and so they will affect and disturb the flow over a larger area of the nose cone. The airflow will also flow into these pockets producing eddy flows which will again affect the airflow over the nose cone. Engine testing and analysis has shown that disturbances to the airflow over the nose cone reduce the efficiency of the root portions of the fan downstream of the nose cone. This in turn has a significant effect on the overall engine performance.
Additionally the fabrication of a nose cone incorporating pockets is complex and therefore expensive. The pockets within the nose cone, and to a certain extent the holes, increase the stresses within the nose cone requiring that the nose cone has to be made from thicker material. This undesirably increases the weight of the nose cone.
Nose cone assemblies in which a fairing surrounds part of the nose cone have been proposed. Whilst the use of a separate fairing may reduce some of the aerodynamic difficulties associated with the arrangements described above, it increases the complexity and therefore cost of the resultant nose cone assembly.